1. Technical Field
The subject matter of this invention relates to radial pneumatic tires for vehicles and particularly to the design and configuration of an improved belt package.
2. Discussion of the Art
The modern day tire must be durable, perform well and be resistant to wear, as well as light in weight and economical to construct. The ability of vehicles to perform at high rates of speed and to have excellent cornering stability remains a desirable feature to the consumer. The tire must produce the higher forces to achieve this performance and at the same time have a smooth ride and absorb shock loads from bumps and holes in the roadway. Another desirable feature of the tire is to resist the penetration of water into the belts which produces corrosion of the reinforcing materials.
Conventional radial tires use multiple belt plies with steel reinforcing materials to achieve vehicle performance. For most applications the tire is sufficiently strong using two steel belt plies. However, these steel reinforced belt plies have a higher mass, do not absorb shock loads very well and are subjected to higher corrosion. One solution to reduce the tire's weight, improve its shock absorbing ability and to make it more resistant to corrosion is to use a high modulus textile cord for the reinforcing members in one or more belt plies. An aromatic polyamide cord material has been used to replace steel reinforcing members in some tires with limited success. Improvements in rigidity (stiffness), riding comfort, low cost of manufacture, tire weight, corrosion resistance and belt edge separation have been noted with different designs. However, the need remains to make these improvements to tires even greater; especially to increase their high speed and impact endurance, to increase their ability to resist corrosion, to help them maintain good wear rates and to maintain their relatively low weight. Various designs are discussed below.
Tires having multiple belt plies with at least one ply having aromatic polyamide reinforcing members are disclosed in U.S. Pat. Nos. 3,851,693; 4,184,529; 4,184,530; and 4,407,347 as well as French Patent No. 2,536,018 and Japanese Patent No. 2-136,304. More than two belt plies are disclosed by these patents. Such constructions are of interest in the art as showing the use of an aromatic polyamide material for a belt ply reinforcment.
U.S. Pat. No. 4,184,529 teaches a belt package for an off-road tire used on heavy vehicles. This belt package has three belt plies reinforced with steel cables and the possibility to use synthetic textile reinforcing members is mentioned.
The reinforced belt package of U.S. Pat. No. 4,184,530 includes a single inner belt ply having aromatic polyamide reinforcing members and a single outer belt ply having metallic reinforcing members; plus two belt edge strips having textile reinforcing members. The angle of the aromatic polyamide belt ply reinforcing members from the mid-plane is less than the angle of the steel belt ply reinforcing members. Other patents also show the inner belt having aromatic polyamide reinforcing materials. In U.S. Pat. No. 4,407,347 two inner belt plies preferably have aromatic polyamide reinforcing members disposed at 15 to 30 degrees plus an outer belt with steel reinforcing members disposed at 60 to 90 degrees. The belt package further includes two lateral strips that adjoin the steel reinforced belt ply at both edges.
Two additional patents which also include a belt package with auxiliary layers or cover plies are French Patent 2,536,018 and Japanese Patent 2-136,304. These patents use an inner belt ply having steel reinforcing members a middle belt ply having aromatic polyamide reinforcing members and an outer cap ply of nylon or polyester reinforcing material at 0 to 10 degrees. A wide range of angles is given for the reinforcing materials in each of the two inner plies, being from 0 to 35 degrees, and the angles in respective plies are not the same. The claims of 2-136,304 are further discussed below.
A number of patents disclose the use of a belt ply comprising aromatic polyamide reinforcing members where one or more of the belt plies are folded to give improved endurance of the belt package. These include U.S. Pat. Nos. 3,949,797 and 4,854,360 and French Patent Nos. 2,235,810 and 2,472,484. The use of folded belts allows certain performance improvements, however, the cost of making tires with folded belt packages is increased by their relatively complex construction requirements. Also, these tires have greater mass as a result of the folded belt plies.
A number of patents illustrate only two unfolded belt plies in the belt package with one belt ply having aromatic polyamide as a reinforcing material. French Patent No. 2,248,161 discloses a belt package with a single outer belt ply having steel reinforcing members and a single inner belt ply having aromatic polyamide (aramid) reinforcing members. The angles of the reinforcing members are briefly mentioned in the disclosure as being "normal" values, without further definition. The aramid belt ply is wider than the steel belt ply in this disclosure. In U.S. Pat. No. 4,936,366 the single unfolded aromatic polyamide reinforced belt ply can be the inner belt ply or the outer belt ply, and the same is true with the other unfolded steel reinforced belt ply. This patent requires the angles of the steel reinforcing members to be greater than the angles of the aromatic polyamide reinforcing members by 1 to 6 degrees. This angular relationship is specified to substantially equalize the bending stiffness between the two belt plies. This stiffness relationship should allow the two belt plies to be interchanged. Angles of the steel reinforcing members are in a range of 18 to 25 degrees where the angles of the aromatic polyamide reinforcing members are in a range of 17 to 24 degrees. The width of the inner belt ply may be equal to the width of the outer belt ply with both equal to or slightly greater than the tread width. The disclosed breaking strength of the 2.times.0.30 millimeter high strength steel reinforcing members is much lower than the disclosed braking strength of the 1670.times.3 aromatic polyamide reinforcing members. Also, the pace (spacing) of the steel reinforcing members in U.S. Pat. No. 4,936,366 is disclosed to be less than the pace of the aromatic polyamide reinforcing members.
In U.S. Pat. No. 3,851,693 all of the examples described in the specification (no drawings) concern three belt plies consisting of two conventional steel reinforced layers and one textile reinforced layer (rayon or aromatic polyamide). The steel reinforcing members are at 15 degrees from a mid-plane and the textile reinforcing members are at 32 degrees from this same plane. For future discussions in this specification the angles are assumed to be measured relative to the mid-plane unless otherwise specified. A further discussion of the claims of this patent follows.
Two patents illustrate belt packages with more than two belt plies but claim a tire with only two belt plies. In U.S. Pat. No. 3,851,693 two steel reinforced belt plies and one aromatic polyamide reinforced belt ply are described and compared in the examples. In claim 1 only a single steel belt ply is claimed with steel reinforced belt members having an angle of 5 to 25 degrees and the other ply has aromatic polyamide reinforcing members having an angle of 0 to 45 degrees. Angles used in the examples were 15 degrees for the steel members and 32 degrees for the aromatic polyamide members. Specific values for the steel cable twist coefficient, modulus of the reinforcing members and the modulus of the rubber at 300 percent elongation are claimed parameters to achieve high speed performance. The tenacity, modulus and twisting coefficient for the aromatic reinforcing members are also a part or this claimed invention. Such material parameters are narrow in scope in comparison to the very broad scope when using both material properties and stiffness variations which result from varying the number of the reinforcing members per unit width in each belt ply. Individual stiffness values or relative stiffness properties between the two belt plies are not design parameters used for the tire of this reference.
Japanese Patent No. 2-136,304 also has only examples of tires with three belt plies, including a cap ply having nylon reinforcing members. This patent claims a belt package with two plies; one being a single inner belt ply reinforced with steel members and the other a single outer aromatic polyamide reinforced belt ply. The first angle of the steel reinforcing members is from 0 to 30 degrees and the second angle of the aromatic polyamide reinforcing members is set to be larger than the first angle and also in a range of 0 to 30 degrees. The angles used in an example are 22 degrees for the steel reinforcing members and 30 degrees for the aromatic polyamide reinforcing members. The patent is silent in regard to the pace of the reinforcing members, but the plies are designed such that the in-plane and out-of-plane stiffnesses of the aromatic polyamide reinforced belt ply are less the same stiffnesses of the steel reinforced belt ply that it replaces. To compensate for this and maintain the in-plane bending rigidity of the belt package the angle of the steel reinforcing members is made less than the angle of the aromatic polyamide reinforcing members. The belt package stiffness also relies on the "triangular truss structure" made possible by the nylon cap ply used in the examples.
Whereas the art defines reinforcing member angles over a very wide range of values, no specific information is provided on the density of the belt reinforcing members (pace) to achieve specific belt ply strength and stiffness values and parametric relationships between the two plies. The need exists to define the structural parameters of a steel reinforced inner belt ply along with an aromatic polyamide reinforced outer belt ply in sufficient detail such that the tire has less weight and continues to perform as well as or better than a tire with two steel reinforced belts. This includes defining the physical properties of the belt reinforcing and rubber matrix materials as well as the density of reinforcing members in each ply, their angles from the mid-plane of the tire and stiffness values for the two belt plies and the belt package.